1. Field of the Invention
The present invention relates to a method for producing an n-type group III nitride single crystal, an n-type group III nitride single crystal, and a crystal substrate.
2. Description of the Related Art
Currently, InGaAlN-type (group III nitride) devices used as a light source to emit UV light, purple light, blue light, and green light are mostly produced by a production method that includes a step of growing a group III nitride single crystal on a sapphire substrate or a silicon carbide (SiC) substrate by MO-CVD (metal organic chemical vapor deposition), MBE (molecular beam epitaxy) or the like.
However, when a sapphire substrate or a SiC substrate is used as a substrate, there is the problem that the crystal has many defects due to the large difference in the thermal expansion coefficient and in the lattice constant between the substrate and the group III nitride. Consequently, this can cause a deterioration in the device characteristics, such as a shortening in the life of the group III nitride device, which is a light-emitting device. Further, this can also cause the operating power to increase.
To resolve these problems, it is best to use the same material for the substrate and for the crystal to be grown on the substrate, and to grow the crystal on a group III nitride single crystal substrate such as a gallium nitride (GaN) substrate, for example.
In a conventional GaN substrate production method, a thick GaN film is grown by hydride vapor phase epitaxy (HVPE) on an underlayer substrate formed from a different material, such as a sapphire or GaAs substrate, and then the thick GaN film is separated from the underlayer substrate, whereby a GaN substrate having a diameter (φ) of about two inches is produced.
However, in HVPE, since a GaN single crystal is heteroepitaxially grown on the underlayer substrate formed from a different material, there can be unavoidable differences in the thermal expansion coefficient or lattice mismatches between the GaN single crystal and the underlayer substrate. Therefore, a GaN substrate produced by HVPE suffers from having a high dislocation density of about 106 cm−2 and that the GaN substrate can become warped due to the difference in thermal expansion coefficient with the underlayer substrate. Therefore, there is a need for a production method that can achieve further improvements in the quality of a GaN substrate.
As a method to produce a high quality GaN substrate, research and development is being carried out into flux methods, in which a GaN single crystal is grown by dissolving nitrogen in a mixed melt of sodium (Na) and gallium (Ga). In a flux method, the GaN single crystal can be grown at a relatively low temperature of 700 to 900° C., and the pressure in the reaction vessel is also relatively low, at about 100 kg/cm2. Therefore, a flux method is a practical method for producing GaN single crystals.
In Chemistry of Materials, Vol. 9 (1997) 413-416, an example is reported in which, using sodium azide (NaN3) and Ga as raw materials, a GaN single crystal is grown by sealing nitrogen in a stainless steel reaction vessel and maintaining the reaction vessel at a temperature of 600° C. to 800° C. for 24 to 100 hours. Further, Japanese Patent Application Laid-open No. 2008-094704 discloses a method for producing a large crystal of GaN by using a flux method, in which a columnar crystal of GaN is grown using a needle-like crystal of aluminum nitride (AlN) as a seed crystal. In addition, Japanese Patent Application Laid-open No. 2006-045047 discloses a method for producing an AlN needle-like crystal to be used as a seed crystal. Thus, producing a large crystal of GaN by growing a seed crystal by using a flux method is already a well known technique.
Meanwhile, when using a GaN crystal as a substrate for an optical device, since it is necessary to form an n-side ohmic electrode on the GaN substrate, an n-type GaN semiconductor crystal having an n-type carrier concentration of 1017 cm−3 or more is required. Therefore, growing an n-type GaN crystal by using a flux method by adding (doping) a donor such as oxygen or germanium in the GaN crystal is being investigated.
However, in Japanese Patent No. 4223540 and Japanese Patent Application Laid-open No. 2010-1209, there are the problems that when the doping amount of germanium is increased, the crystal growth rate decreases and the device characteristics deteriorate due to increased absorption of visible light.
Concerning the addition of oxygen, for example, Japanese Patent Application Laid-open No. 2005-154254 discloses a technique in which, using sodium oxide (Na2O) and oxygen gas as the dopants, about 2×1017 cm−3 of oxygen is doped in a group III nitride crystal. Further, Japanese Patent Application Laid-open No. 2007-246303 discloses a technique in which about 1018 to 1020 cm−3 of oxygen is doped by including oxygen and moisture in the atmospheric gas in a glove box when preparing the raw materials.
Concerning germanium addition, for example, Japanese Patent No. 4223540 discloses a technique in which about 2×1019 cm−3 of germanium is added to a group III nitride. Further, Japanese Patent Application Laid-open No. 2010-001209 discloses a technique for producing a low-resistance GaN crystal having an electron concentration of about 5×1019 cm−3 by simultaneously adding carbon and germanium to dope 2×1017 cm−3 or more to 1×1020 cm−3 or less of germanium.
However, Japanese Patent Application Laid-open No. 2005-154254 suffers from the problem that a crystal having a sufficiently large carrier concentration cannot be obtained, which makes it difficult to form a low-resistance ohmic electrode. Further, in Japanese Patent Application Laid-open No. 2007-246303, there is the problem that the production processes are complex, such as need to enclose oxygen and moisture in the reaction vessel in the glove box and seal the vessel. Moreover, even though the oxygen and moisture may only temporarily cause the atmosphere in the glove box to deteriorate, there is the problem that the life of the catalyst which removes the oxygen and the moisture is shortened, which makes it expensive to mass produce the crystal.
Thus, the conventional art suffers from the problem that it is difficult to produce a crystal having a large carrier concentration by efficiently doping oxygen into the crystal because the oxygen is doped using a gas or an oxide that has a higher melting point than the crystal growing temperature. Further, there is also the problem that to obtain a crystal having a large carrier concentration, the apparatus and the production steps become complex.